The present invention relates to a method for treating the surface of a thermoplastic resin article and, in detail, relates to a surface-treating method for improving coating performance, printability, adhesiveness and the like, when a molded article having a three-dimensional shape, made by injection molding and the like, or a sheet type article, made by extrusion molding and the like, is subjected to a coating process, a printing process, or a process of adhering it to another article.
Hitherto, as for a surface-treating method for improving coating performance, printability, adhesiveness and the like of a thermoplastic resin article there is a method, such as a corona discharge treatment, a plasma treatment, an ultraviolet-irradiation treatment and the like, which comprises forming active species on a surface of said resin article and, by an action of the active species, improving bonding performance with a coating, ink, an adhesive and the like. In .addition, there is a method for improving the bonding performance by applying a primer (an undercoating) before applying a coating and the like.
Improving effects of the aforementioned various kinds of activation treatments or the primer treatment upon coating performance and the like are very much affected by surface conditions of a resin article to be treated.
Practically, when a resin article is molded, an antioxidant, a plasticizer or the like contained in the resin article bleeds out on the resin article surface to deteriorate conditions of the resin article surface. In some cases, a mold-release agent or an anticorrosive oil may attach to the resin article surface and damage conditions of the resin article surface. In case of using a metal mold for injection molding and the like, when the mold is released, microscopically fine flaws may be given to the resin article surface and said flaws may deteriorate conditions of the resin article surface. In molding a resin article according to the aforementioned, a part of the resin article surface, of which performance is inferior to that of other parts, may be called WBL (Weak Boundary Layer) .
Also, after a resin article has been molded, while storing or handling to transport it, there is a case where the surface becomes dirty or suffers flaws due to a touch with others, so that surface conditions of the part becomes bad. Concretely, for example, when a resin article is transferred by a conveyor and the like, there is a possibility that the resin article suffers flaws or becomes dirty due to a contact with a roller or a guide. Also, there are many cases where a sheet article having a certain extent of thickness is stored or handled under piled conditions and where a relatively thin film article is handled under wound conditions. In any of the cases, the article surfaces are apt to suffer flaws resulting from touching with each other. Furthermore, when the resin article surface is cleaned or washed, a brush roller and the like occasionally give flaws.
Especially, in case of an article molded by injection molding, burrs are occasionally formed at a periphery part, a hole part or other parts of the molded article. To repair the part, an operation to remove the burrs by sand paper of about #800 to #1200 is carried out, and traces by this sanding causes problem similar to that by said flaws and the like and poor adhesion may occur.
In case that there is said WBL part being formed in molding the resin article or that there are flaws, dirt, sanding traces or the like being formed in the course of handling the resin article, even if an activation treatment, such as a corona discharge treatment and the like, or a primer treatment is carried out, treatment effects are not sufficiently obtained and bonding performance in coating or printing being applied after said treatment become poor.
Accordingly, there have been proposed various methods for further improving coating performance and the like of the resin article by improving effects of such a treatment as a corona discharge and the like.
For example, in Japanese Official Patent Provisional Publication No. showa 61-151244 there has been proposed a method which comprises, before the plasma treatment, carrying out heating by blowing a hot wind or by using an oven, or carrying out high pressure air-blowing, to remove a solvent remaining on the surface of a molded resin article.
In Japanese Official Patent Provisional Publication No. showa 59-221336 there has been disclosed a method which comprises treating by low temperature plasma under conditions where a thermoplastic resin is heated at its melting or softening point or at a higher temperature to improve effects of the low temperature plasma treatment, and thereby, improving coating performance, printability and the like of the thermoplastic resin.
On the other hand, it is typical to carry out a primer treatment before applying the coating or printing to a thermoplastic resin article. Hitherto, as the primer treatment in many cases, a washing treatment by a chlorine-based solvent such as trichloroethane is carried out before coating a primer. The washing treatment by a chlorine-based solvent is said to improve effects of the primer treatment more than, for example, a washing treatment by a water base washing agent.
Also, it proves that, if a washing agent similar to the above-mentioned which comprises a chlorine-based solvent including trichloroethane and the like is used before applying a corona discharge treatment or the like to a thermoplastic resin article, there are some cases where effects of treatment such as corona discharge and the like are enhanced.
However, any one of the aforementioned conventional methods could not sufficiently improve effects of such a treatment as corona discharge and the like.
For example, according to a method in the Japanese Official Patent Provisional Publication No. showa 61-151244, a solvent and the like which have attached to or been impregnated into the surface of a molded resin article can be removed to some extent. However, there cannot be solved the defects of surface conditions resulting from flows or sanding traces on the surface or from internal constitution of the molded resin article surface or from a surface structure itself.
According to a method of the Japanese Official Patent Provisional Publication No. showa 59-221336, since active species formed on the surface of a thermoplastic resin by contact with plasma is taken into an inside of the resin which is in a fluid condition by heating to melt it, active species remaining on the resin surface after the resin being cooled and solidified are in many cases merely a part of active species formed by the plasma, so that effects of the plasma treatment are diminished. The defect in this method is also found in the case where a thermoplastic resin article, for example, a film is treated by extrusion molding and, before this film being cooled and solidified and under a surface-melted condition, it is subjected to corona discharge.
Next, since it is inferred that the environment may be destroyed by using a washing agent containing a chlorine-based solvent, such as trichloroethane, flon and the like, as a washing agent for a pre-washing process in the primer treatment, use of a washing agent for the primer treatment has been required to discontinue it. If a water base washing agent is used, the problem relating to the environment is only a few. However, a problem for the water base washing agent is that attaining sufficient effects of the primer treatment is difficult and improving effects on the coating performance or printability are insufficient. An entirely similar problem is found in the case where washing treatment being performed before corona discharge and the like is carried out by using a water base washing agent in place of a washing agent comprising a chlorine-based or non-chlorine-based solvent.